The present invention relates to an image forming apparatus of the type configuring a toner, or developer, into a magnet brush for developing an electrostatic latent image and, more particularly, to the structure of a toner cartridge which contains the toner and is removably mounted on the image forming apparatus.
In an electrophotographic copier, facsimile apparatus, printer or similar image forming apparatus of the type described, a developing unit generally uses a two-component developer which is made up of carrier particles and toner particles which are deposited on the carrier particles. The apparatus includes a developing sleeve in which magnets are accommodated to cause the developer to form a magnet brush on the sleeve. The magnet brush is brought into contact with a latent image electrostatically formed on a photoconductive element, or image carrier, so that only the toner of the developer is deposited on the latent image to form a toner image. Since the toner concentration or density in the developer sequentially decreases due to the repetitive copying operation, it is necessary to supply a fresh toner when the decrease in toner density is sensed. Usually, a toner is contained in a toner cartridge which is mounted on the developing unit of the image forming apparatus. Hence, when the toner cartridge runs out of toner, it has to be replaced with a new toner cartridge which is filled with a fresh toner.
It is a common practice to set the toner cartridge in a cartridge container which is provided in the developing unit and, when the density of a toner image formed on the photoconductive element is determined to be low, to cause the toner to be supplied through an opening of the cartridge into the casing of the developing unit. When the toner concentration in the cartridge decreases beyond a predetermined threshold as a result of repeated tone supply, a toner end state is sensed and displayed or otherwise indicated to alert the operator to such an occurrence. In practice, however, even after the operator has been warned of the toner end state, some toner is still available in the developing unit so that the development may be repeated for a while, although the image density will be somewhat lowered. Therefore, it sometimes occurs that the operator continuously uses the copier in spite of the toner end warning by resetting a toner end mode intentionally without replacing the toner cartridge, at the sacrifice of image density. This brings about a problem that, apart from the decrease in image density, silicon films provided on the carrier particles are destroyed due to the exhaustion of toner to lower the insulation resistance, resulting in the deposition of the carrier particles on the photoconductive element.